Microelectromechanical devices (referred to as MEMS devices) are becoming commonplace in modern technology such as smartphones, smartwatches, and picoprojectors. Of particular concern with MEMS devices is that their behavior remain consistent or constant over a range of temperature. This in turn requires that oscillators used to generate clock signals used by such MEMS devices provide a stable and constant frequency output over a range of temperatures.
While a common quartz crystal is capable of providing this function, typical MEMS devices lack the available space to include a quartz crystal. Therefore, there is a need for on-chip clock signal generation by a non-crystal based oscillator that is capable of providing the required stable and constant frequency output over a range of temperatures.
One prior approach has been to design an RC oscillator utilizing resistors constructed from materials that have a resistance that remains constant over temperature. However, this temperature independence tends to be only under common operating conditions, and in some conditions, the resistance may vary undesirably, leading to change in frequency of the generated clock signal. To address this, such prior approaches utilize resistor trimming circuits.
While this trimming can be effective, it requires testing under different temperatures in order to tune the trimming circuit properly. Unfortunately, such testing at temperature is difficult in a production environment, as the thermal cycling involved consumes a large amount of time and is costly. In addition, the measurement of the temperature of a chip itself is difficult. Also, moving wafers between different pieces of equipment is difficult and costly. Compounding all of this is the fact that measurement error may be introduced depending on equipment calibration.
As a consequence, new designs for non-crystal based oscillators capable of providing stable and consistent output over temperature are needed. Also, quicker and less expensive methods of testing such oscillators are needed.